Method for steering a vehicle with superimposed steering

ABSTRACT

A method of steering a vehicle with a superimposed steering system, wherein a steering angle input by the driver and an additional angle (additional steering angle) is determined and wherein the additional steering angle can override the input steering angle according to further quantities, in particular diving-dynamics quantities, by means of an electric motor, is characterized in that the method includes a steering angle control with a subordinated current or torque control of the electric motor.

TECHNICAL FIELD

The invention relates to a method of steering a vehicle with a superimposed steering system, wherein a steering angle input by the driver and an additional angle (additional steering angle) is determined and wherein the additional steering angle can override the input steering angle according to further quantities, in particular diving-dynamics quantities, by means of an electric motor.

BACKGROUND OF THE INVENTION

Up-to-date motor vehicles, in particular passenger vehicles, are generally equipped with hydraulic or electrohydraulic servo steering systems, wherein a steering wheel is compulsively coupled mechanically with the steerable vehicle wheels. The servo assistance is devised such that actuators, e.g. hydraulic cylinders, are arranged in the mid-portion of the steering mechanism. A force generated by the actuators assists in the actuation of the steering mechanism in response to the turning of the steering wheel. This reduces the force the driver has to apply during the steering operation.

Superimposed steering systems are known in the art. They are characterized in that the steering angle input by the driver can be overridden in case of need by another steering angle (additional steering angle) by means of an actuator. Usually electric actuators are employed which act on an overriding drive and adjust the additional steering angle largely independently of the driver.

The additional steering angle is controlled by an electronic controller and is e.g. used to increase the stability and agility of the vehicle. According to a prior art control concept, as described in DE 197 51 125 A1, the steering components of the superimposed steering angle are produced irrespective of each other.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to provide a method of steering a vehicle with a superimposed steering that is safe and reliable in operation.

According to the invention, this object is achieved in that the method includes a steering angle control with a subordinated current or torque control of the electric motor.

To this end, a nominal current or a nominal motor torque is produced by means of which the electric motor introduces an additional steering angle into the steering system. Due to the angle superimposed on the steering actuation, the desired steering angle and, hence, also the additional steering angle is adjusted, which latter is additionally demanded by other vehicle control systems, as the case may be.

It is arranged for in the invention that an actual steering angle value and a nominal steering angle value is determined and, according to a comparison between the actual steering angle value and the nominal steering angle value, a nominal current or a nominal motor torque is produced by which the electric motor introduces the additional steering angle into the steering system.

A favorable embodiment of the method of the invention includes that a steering request of the driver δ_(DRV) is determined on the basis of a steering wheel angle δ_(H) adjusted by the driver, wherein the driver's steering request δ_(DRV) is composed of the adjusted steering wheel angle δ_(H) and an invariably or variably predeterminable gear ratio factor and the gear ratio factor is chosen corresponding to the current driving situation, in particular a detected longitudinal vehicle speed and/or a steering wheel turning angle, and that a nominal steering angle value δ_(nominal) is determined on the basis of the so calculated steering request of the driver and sent to the steering control.

According to another embodiment of the invention, the driver's steering angle δ_(H) is determined and, in connection with a gear ratio factor i_(L1) by which the driver's steering angle acts directly on the steering gear, an additional steering angle δ_(M) is additively superimposed thereon in connection with a second gear ratio i_(L2), and a superimposed steering angle δ_(L) is determined and sent as an actual value δ_(L,actual) to the steering control, with said superimposed steering angle δ_(L) being determined according to the following formula: δ_(L) =i _(L1)*δ_(H) +i _(L2)*δ_(M).

The invention provides that a driving dynamics control (ESP system) cooperates with the steering control and that an additional steering angle Δδ responsive to driving dynamics is determined when the necessity of a stabilizing intervention is detected by driving dynamics control.

Preferably, the additional steering angle Δδ responsive to driving dynamics that is produced on the basis of a correcting intervention of a driving dynamics controller is additively superimposed on the driver's steering request δ_(DRV).

The control of the superimposed steering is improved by this embodiment of the method of the invention in particular in highly dynamic driving situations. The term ‘highly dynamic driving situation’ refers to all driving situations with a relatively quick change of the vehicle direction and/or the vehicle speed, which can cause instability of the vehicle or the desired vehicle movement. Driving situations in the frontier of driving dynamics, such as skidding maneuvers, demand too much from many drivers regarding a suitable steering performance.

It is arranged for by the invention that based on the series steering ratio i_(L,series) and due to a boosting factor K1 responsive to a steering wheel angle and a boosting factor K2 responsive to the vehicle speed, a resulting steering ratio I_(L,ESAS) which corresponds to the ratio between the steered wheels δ_(V) and the driver's steering angle δ_(H) is determined according to the following formula: i _(L,ESAS)=δ_(V)/δ_(H) =i _(L,series)/(K1*K2)

According to the invention, an anticipatory control of the nominal speed of the motor ω_(M,nominal) is executed, which is determined from a motor speed specification ω_(M,spec) and a motor speed preset value ω_(M,reg), and the motor speed preset value ω_(M,reg) is determined on the basis of a comparison between a nominal steering angle value δ_(L,nominal) and a determined actual steering angle value δ_(L,actual), and the motor speed specification ω_(M,spec) is determined from the time derivative of the nominal steering angle value δ_(L,nominal) and the driver's steering angle δ_(H) and a gear ratio factor i_(L2) by means of the following formula: ω_(M,spec)=({dot over (δ)}_(L,nominal) −i _(L1){dot over (δ)}_(H))/i _(L2).

According to the invention, the control of the motor of the superimposed steering is realized by a computer program which includes appropriate program steps for implementing the described method.

The above object is also achieved by a steering system for a vehicle, comprising a steering wheel arranged at a steering column, a steering gear, a steering angle sensor arranged at the steering column, an overriding motor that acts on the steering column by way of an overriding gear, an electric steering control element, a sensor for measuring the position of the steered wheels, and a steering control device, in which steering system the steering control device includes a means for implementing the method of the invention described hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a block diagram of the basic structure of the method of the invention.

FIG. 2 is a block diagram of the structure of the method of the invention.

FIG. 3 is a block diagram for determining a nominal value and an actual value as input quantities of the steering angle control according to the invention.

FIG. 4 is a block diagram for determining a nominal value and an actual value as input quantities of the steering angle control according to the invention.

FIG. 5 is a block diagram for determining a motor torque specification for the electric motor for adjusting the overriding angle according to the invention.

FIG. 6 is a block diagram for determining a field weakening current and a nominal current for actuating the electric motor according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basis structure of the method of the invention is represented in FIG. 1.

Based on the steering wheel angle δ_(H) 50 adjusted by the driver, the driver's steering request δ_(DRV) 52 is calculated in the basic steering function as a nominal steering angle value 53 δ_(L,nominal) (input quantity) for the steering control circuit 54 by way of a variably or invariably predeterminable gear ratio i_(L,ESAS) 51. In this arrangement, the basic steering function generally comprises the selection of a steering ratio i_(L,ESAS) corresponding to the current driving situation, e.g. the detected longitudinal vehicle speed. The actuator of the steering system is then driven corresponding to a steering angle δ_(L) 55 (output quantity of the control circuit 54).

Driving stability and agility of the vehicle can be enhanced by means of adapting the position of the steered wheels, principally irrespective of the driver's request. To this end, an additional steering angle Δδ 56 responsive to driving dynamics is additively superimposed 58 on the driver's steering request δ_(DRV) 52 on the basis of a correcting intervention of a driving dynamics controller 57. The result is the nominal steering angle value δ_(L,nominal).

FIG. 2 shows the structure of the method. The driver's steering angle δ_(H) acts in the overriding gear as an input quantity 1 by way of a mechanical gear 2 with a gear ratio factor i_(L1) directly on the steering gear 3 (i_(L1)*δ_(H)) 19. The additional steering angle δ_(M) 16 adjusted by a motor acts by way of a second gear 17 with a gear ratio factor i_(L2) and is additively superimposed on the geared steering angle of the driver: δ_(L) =i _(L1)*δ_(H) +i _(L2)*δ_(M).

The steering gear 3 generates as an output quantity a resulting steering angle δ_(V) that acts upon the vehicle.

The driving dynamics of the vehicle 5, especially the yaw torque about the vertical axis of the vehicle 5, and the transverse acceleration are determined. The driving-dynamics quantities 7 and the driver's steering angle δ_(H) 8 are sent as input quantities to a driving dynamics controller 6. Driving-dynamics-related steering interventions in the capacity of an additional steering angle Δδ 9 are sent as an input quantity to a steering controller 10 by means of the driving dynamics controller 6. Likewise, the driver's steering angle δ_(H) 11 and a value for the present vehicle speed 12, in particular the vehicle reference speed from the driving dynamics controller 6 or an ABS controller, is sent as an input quantity to the steering controller 10. Said steering controller 10 drives the actuator 14 of the overriding steering function 15.

The actuator, in particular an electric motor 14, produces an additional steering angle δ_(M), which acts by way of a gear 17 with a gear ratio factor i_(L2) on the steering gear 3 (i_(L2)*δ_(M)) 18. Gear 2 and gear 17 are illustrated herein as two individual ‘gears’ only for representation purposes. However, the two gear ratios of gears 2 and 17 are preferably realized by way of one single gear unit, in particular a planetary gear.

As can be taken from FIG. 1 already, the additional steering angle Δδ which shall be considered as an external intervention of the driving dynamics controller 6 is additively superimposed at 58 on the nominal steering angle δ_(DRV) of the basic steering function. The nominal steering angle value δ_(L,nominal) resulting from this addition is sent to the control of the superimposed steering.

A sum steering angle δ_(L) 21 is the result of the additive superposition of driver's steering angle and superimposed steering angle generated by the actuator, from which sum steering angle a resulting steering angle δ_(V) is produced by the steering gear 3 as a resulting output quantity and acts on the vehicle corresponding to the desired learning function.

The sum steering angle δ_(L) 21 is furnished to the steering controller 10 as an input quantity at 22, just as the additional steering angle δ_(M) 23. The sum steering angle δ_(L) 21 is also sent 26 as an input quantity to the driving dynamics controller 6. Signals or measured quantities of the actuator means, the electric motor 14, are also sent to the steering controller 10 at 24.

FIG. 3 shows the determination of the nominal steering angle value δ_(L,nominal) and, if needed, a motor speed specification ω_(M,spec) 44 in a nominal value producing means 30 and the determination of the actual value δ_(L,actual) in an actual value producing means 31, said values being used as input quantities 32, 33 of the steering controller 34 under consideration. A motor torque M_(mot,nominal) 35 to be adjusted or a torque-producing motor current l_(q,nominal) is produced from output quantities. These quantities are associated with the electric motor, exactly as a commutation of the motor (in the case of an electronic commutation).

In this arrangement, the control quantity of the steering controller 34 is the steering angle δ_(L), which is either directly measured and sent 36 to the actual value producing means 31, or which can be calculated in the actual value producing means 31 by means of the motor angle δ_(M) 37 and the driver's steering angle δ_(H) 38 in consideration of the gear ratio of the overriding gear. The motor speed ω_(M,actual) 40 which can be calculated from the measured motor angle by differentiation is used as internal control quantity.

The driver's steering angle δ_(H) 41 and the additional steering angle Δδ 42 and the vehicle speed V_(VEH) 43 are also sent to the nominal value producing means.

FIG. 4 shows the determination of the nominal steering angle δ_(L,nominal) 32 in greater detail.

The resulting steering ratio i_(L,ESAS) 60 corresponds to the ratio between the angle of the steered wheels (wheel turning angle) δ_(V) and the driver's steering angle δ_(H). It results from two boosting factors K1 61 and K2 62 which are multiplicatively combined with the series steering gear ratio i_(L,series) by the following formula: i _(L,ESAS)=δ_(V)/δ_(H) =i _(L,series)/(K1*K2)

The boosting factors represent a component K1 responsive to the steering wheel angle 63 and a component K2 responsive to the vehicle speed 64. They can be chosen freely according to aspects related to driving dynamics or specifications by the driver. To calculate the nominal steering angle value δ_(L,nominal) and the motor speed specification ω_(M,spec) 66, the additional steering angle Δδ 67 is also taken into consideration, and a corrected additional steering angle Δδ_(IPO) 69 is superimposed at 71 on the driver's request δ_(nominal,DRV) 70 after an interpolation and limitation of rise 68.

The motor speed specification ω_(M,spec) 66 is calculated from the time derivative of the nominal steering angle value δ_(L,nominal) and the steering angle of the driver δ_(H) by the following formula 72: ω_(M,spec)=({dot over (δ)}_(L,nominal) −i _(L1){dot over (δ)}_(H))/i _(L2).

FIG. 5 shows the steering angle control in greater detail. Said control is a cascade control in its basic structure. An anticipatory control of the nominal speed of the motor is executed to enhance the dynamics of the control circuit. The nominal speed ω_(M,nominal) is produced 83 from the motor speed specification ω_(M,spec) 81 and the motor speed preset value ω_(M,reg) 93 being determined as an output quantity of the angle controller based on the comparison between the nominal steering angle value δ_(L,nominal) and the actual steering angle value δ_(L,actual) determined. To prevent impairment of the steering comfort by the anticipatory control especially during slow steering movements, the anticipatory control value is weighted depending on the desired motor speed at 83, 84.

The nominal motor torque M_(mot,nominal) 86 or a torque-producing nominal motor current I_(q,nominal) 87 by which the motor shall be driven, is produced from the nominal speed ω_(M,nominal) 80 and the comparison with the actual motor speed ω_(M,actual) 88 determined by way of a motor speed controller 85.

A higher motor speed than available may be required in certain cases of operation. In this case, a demand-responsive brief increase of the motor speed without reduction of the available motor torque can be reached by using a field weakening. A brief increase of the current consumption is related thereto. In particular the existence of a very direct steering ratio and a high nominal speed on the part of the driver or the driving dynamics control system is considered as a case of need. The resulting controller structure represents an extension of the structure shown in FIG. 5 and is illustrated in FIG. 6. Therefore, all steps and elements corresponding to FIG. 5 have been assigned equal reference numerals in FIG. 6 and will not be explained in detail in the following.

A decision about the use of the field weakening and the magnitude of the field weakening current is taken 104 based on the present actual condition of the steering system, that means the prevailing actual motor speed ω_(M,actual) 100 and the prevailing steering angle value δ_(L,actual) 101 as well as the desired nominal condition, i.e. the motor speed specification ω_(M,spec) 102 and the nominal steering angle value δ_(L,nominal) 103 and the boosting factors of the steering ratio 106. In case field weakening of the motor is not necessary, the resulting field weakening current I_(d,nominal) 105 is zero, i.e. 0 A. The torque control of the electronically commutated motor is then required to control the field-weakening current value Id in addition to the torque-producing current Iq 87. 

1. A method of steering a vehicle with a superimposed steering system comprising; inputting a steering angle by the driver; determining an additional steering angle wherein the additional steering angle can override the input steering angle according to further quantities, through an electric motor; providing a steering angle control with a subordinated current or torque control of the electric motor; executing an anticipatory control of a nominal speed of the motor ω_(M,nominal,) determined from a motor speed specification ω_(M,spec) and a motor speed present value ω_(M,reg,) by: determining the motor speed preset value ω_(M,reg) by comparing a nominal steering angle value δ_(L,nominal) and a determined actual steering angle value δ_(L,Actual,) and determining the motor speed specification ω_(M,spec) from the time derivative of the nominal steering angle value δ_(L,nominal) and the driver's steering angle δ_(H) and a gear ratio factor I_(L2) by means of the following formula: ω_(M,spec)=({dot over (δ)}_(L,nominal) −I _(L1){dot over (δ)}_(H))/I _(L2).
 2. The method as claimed in claim 1, wherein an actual steering angle value and a nominal steering angle value is determined and, according to a comparison between the actual steering angle value and the nominal steering angle value, a nominal current or a nominal motor torque is produced by which the electric motor introduces the additional steering angle into the steering system.
 3. The method as claimed in claim 1, wherein a steering request of the driver δ_(DRV) is determined on the basis of a steering wheel angle δ_(H) adjusted by the driver, and wherein the driver's steering request δ_(DRV) is composed of the adjusted steering wheel angle δ_(H) and an invariably or variable predeterminable gear ratio factor and the gear ratio factor is chosen corresponding to the current driving situation, and wherein a nominal steering angle value δ_(nominal) is determined on the basis of the so calculated steering request of the driver and sent to the steering control.
 4. The method as claimed in claim 1, wherein the driver's steering angle δ_(H) is determined and, in connection with a gear ratio factor i_(L1) by which the driver's steering angle acts directly on the steering gear, an additional steering angle δ_(M) is additively superimposed thereon in connection with a second gear ratio i_(L2), and wherein a superimposed steering angle δ_(L) is determined and sent as an actual value δ_(L,actual) to the steering control, with said superimposed steering angle δ_(L) being determined according to the following formula: δ_(L) =i _(L1)*δ_(H) +i _(L2)*δ_(M).
 5. The method as claimed in claim 1, wherein a driving dynamics control (ESP system) cooperates with the steering control, and wherein an additional steering angle Δδ responsive to driving dynamics is determined when the necessity of a stabilizing intervention is detected by driving dynamics control.
 6. The method as claimed in claim 1, the method further comprising a driving dynamics control (ESP system) cooperates with the steering control, and an additional steering angle Δδ responsive to driving dynamics is determined when the necessity of a stabilizing intervention is detected by driving dynamics control, wherein the additional steering angle Δδ responsive to driving dynamics that is produced on the basis of a correcting intervention of a driving dynamics controller is additively superimposed on the driver's steering request δ_(DRV).
 7. The method as claimed in claim 1, wherein the electric motor is additionally actuated by means of a field weakening current according to further quantities, with a view to increasing the motor speed without reduction of the available motor torque.
 8. The method as claimed in claim 1, the method further comprising the electric motor is additionally actuated by means of a field weakening current according to further quantities, with a view to increasing the motor speed without reduction of the available motor torque, wherein the electric motor is additionally actuated by means of a field weakening current when a very direct steering ratio and/or a high nominal speed is desired or required.
 9. The method as claimed in claim 1, wherein based on the series steering ratio i_(L,series) and due to a boosting factor K1 responsive to a steering wheel angle and a boosting factor K2 responsive to the vehicle speed, a resulting steering ratio I_(L,ESAS) which corresponds to the ratio between the steered wheels δ_(v) and the driver's steering angle δ_(H) is determined according to the following formula: i _(L,ESAS)=δ_(v)/δ_(H) =i _(L,series)/(K1*K2).
 10. A system comprising an electric motor operating under a computer executing computer program instructions encoded on a controller: the system operable to implement the instructions that steer a vehicle with a superimposed steering system, wherein a steering angle input by a driver and an additional steering angle is determined and wherein the additional steering angle can override the input steering angle according to further quantities, through an electric motor, wherein the system includes a steering angle control with a subordinated current or torque control of the electric motor, and where an anticipatory control of the nominal speed of the motor ω_(M,nominal,) is executed in the computer program instructions, the nominal speed of the motor is determined from a motor speed specification ω_(M,spec) and a motor speed present value ω_(M,reg) and the motor speed preset value ω_(M,reg) is determined on the basis of a comparison between the nominal steering angle value δ_(L,nominal) and a determined actual steering angle δ_(L, Acctual,) and the motor speed specification ω_(M,spec) is determined from the time derivative of the nominal steering angle δ_(L,nominal) and the drivers steering angle δ_(H) and a gear ratio factor I_(L2) using the following formula: ω_(M,)=({dot over (δ)}_(L,nominal) −I _(L1){dot over (δ)}_(H))/I _(L2).
 11. A computer executing the instructions set forth in the method as claimed in any one of the claims 1-9. 